Apparatus for effectively reducing overscan in a cathode ray tube



Aug.1, 1967 'SH KAPLAN Em 3,334,258

APPARATUS FOR EFFECTIVELY REDUCING OVERSCAN IN A CATHODE RAY TUBE FiledSept. 4. 1964 INVENTORS Sam H. Kaplan Theodore S. Noskowicz AT RNEYUnited States Patent O 3,334,258 APPARATUS FOR EFFECTIVELY REDUCINGOVERSCAN IN A CATHODE RAY TUBE Sam H. Kaplan and Theodore S. Noskowicz,Chicago, Ill., assignors to The Ranland Corporation, Chicago, Ill., acorporation of Illinois Filed Sept. 4, 1964, Ser. No. 394,563 8 Claims.(Cl. 313-75) This invention is directed in general to cathode ray tubesand, more specifically, concerns increasing the contrast of such tubes.

In cathode ray tube operation, especially in the case of those used fortelevision receivers, it is common practice to overscan the screen areain displaying an image on the tube face. The practice of overscanninghas been adopted in order to compensate for both variations in linevoltage and for the conliguration of the tube envelope itself. It isknown, for example, that variations in line voltage aiect picture sizeand loverscanning assures an image covering the entire screen area evenin the face of the minimum voltage expected. With respect to envelopeconguration overscanning at the corners of the screen is unavoidable ifthe scanning pattern is to till the whole screen area.

A disadvantage of overscanning is loss of contrast since electrons ofthe scanning beam tend to be reflected off the neck and sides of thetube producing secondary electrons and both the primary and secondaryelectrons rain onto the screen in a random pattern.

Electron shields having a maximum diameter less than the diameter ofsaid neck have heretofore been suggested to intercept the electrons andavoid loss of contrast, but they have had many disadvantages. Forexample, some shields have been positioned adjacent the screen area butthis type of shield is relatively inetlicient since most stray electronswhich are generated in the funnel close to the neck of the tube are notintercepted by such a shield. It has also been suggested that a shieldhaving a rectangularly shaped aperture be placed in the funnel adjacentto the neck. This requires, however, that the tube have a removablefaceplate to allow for the placement of the shield which is larger thanthe tube neck. The use of such a shield would be prohibitive for blackand white television because of the attendant cost of the two-pieceenvelope construction. Finally, electron shields featuring a rectangularopening require that the angular orientation of the shields beabsolutely precise. Again, this requirement would significantly increasethe production cost of the tube.

It is, therefore, a major object of this invention to provide a cathoderay tube with improved contrast capability.

It is another object of this invention to prevent the impingement ofstray electrons upon the screen of a cathode ray tube.

It is still another object of the invention to accomplish the above in asimple and economical manner.

The invention is practiced in a cathode ray tube having a beam deectioneld with predetermined center of deflection. In accordance with theinvention a cathode ray tube comprises an evacuated envelope including aneck, a funnel and a substantially rectangular-shaped faceplate havingan image screen thereon. An electron gun is supported within the neckportion for projecting an electron beam along a predetermined path,through the deflection lield, toward the screen. An electron shieldhaving a maximum diameter less than the diameter of the tube neck3,334,258 Patented Aug. 1, 1967 ICC and having a circular aperture issupported by the gun with the shield on the screen side of the center ofdeflection and with the aperture thereof disposed across the path of thebeam and substantially co-axial with the envelope funnel. The positionof the shield and the diameter of the shield aperture are such that thedeflected electron beam is effectively shadowed from the initially aredportion of the envelope funnel and is confined to scanning on the screena pattern having a maximum dimension equal to the diagonal measure ofthe rectangular faceplate.

The features of the present invention which are believed to be novel areset forth with particularity in the appended claims. The organizationand manner of operation of the invention, together with further objectsand advantages thereof, may best be understood by reference to thefollowing description taken in connection with the accompanying drawing,in the -several figures of which like reference numerals identify likeelements, and in which:

FIGURE 1 is a partial schematic diagram showing a cross-sectional viewof a cathode ray tube and associated yoke assembly including the subjectinvention;

FIGURE 2 is a partial cross-sectional view taken along lines 2 2 ofFIGURE 1; and

FIGURE 3 is a perspective view partially cut away of a portion of thegun structure of the cathode ray tube of FIGURE 1.

Referring now to FIGURE l, the cathode raye tube 10 there showncomprises an evacuated envelope having a neck portion 11, a ared conicalor funnel portion 12 and a substantially rectangular-shaped faceplate 13opposite neck 11, which bears a rectangular viewing or image screen. Aphosphor layer 15 is coated on the inside of faceplate 13 and analuminum layer 16 backs phosphor layer 15 to increase the brightness ofthe television picture in a manner well known in the art. A gun assembly17, which in this case consists of a single electron gun, is located inthe neck of the tube and is oriented to project a beam of electronsalong a predetermined path, through a deflection field presently to bediscussed, toward viewing screen 13. As is usually the case, the beampath corresponds with the longitudinal axis or center line of the tubeindicated by dashed line 14. Gun assembly 17 includes a cathode 18,control grids 19 and 20 and accelerating-focusing electrodes 21, 22 and23. All of the individual elements of the gun are retained by supportingposts 25 and the unitary structure is centered in neck 11 by means of apair of snubber springs 26 which are welded to the flange end of nalelectrode 23. The springs 26 also establish a conductive connectionbetween electrode 23 and screen 15 by contacting an internal conductivecoating 27 of colloidal graphite which is painted on the interior offunnel 12.

The detiection system for the tube includes a yoke 28 which encirclesneck 11 and a portion of funnel 12. The yoke has a pair of horizontaldeflection coils 29 and a pair of vertical deflection coils 30,illustrated in FIGURE 2. A cylindrical iron or ferrite core 32 surroundsboth sets of deection coils and the entire assembly is maintained inposition by a retaining ring 51. The yoke 28 also includes anon-magnetic support frame 33 which separates vertical coil 30 fromhorizontal coil 29. As thus far described the cathode ray tube is ofstandard construction and operation.

Numeral 31 indicates the yoke reference line (Y.R.L.)

31 which is a standard reference for a cathode ray tube for determiningthe location of the gun assembly.

The position lon axis 14 at which lateral deflection of the electronbeam is initiated is represented at point 24. The electron beam is ofcourse curved since it passes through -a deflection field of lfinitelength. The maximum deflection is represented by dashed lines 36extending from point 24 to` the edges -of screen 13.

In laccordance with the invention, improvement in contrast is obtainedthrough the use 'of an electrically conductive shield 34 (see FIGURE 3)having a circular, centrally located aperture 34a through which the beampasses on its way to screen 13. Shield 34 is supported by gun assembly17 with the shield positioned on the screen side `of reference point 24yand with the aperture disposed across the beam path. More particularly,shield 34 is attached to final electrode 23 of the gun by means of apair of supports 35 which extend from the flanged portion of the finalelectrode. The location of shield 34 and the diameter of its aperture34u determine the scanning pattern traced by the electron beam 4onscreen 13 in response to the deflection field produced by vyoke 28. Forexample, as the shield is moved toward the screen past deflection point24 its aperture must increase in size to accommodate the normal sweeprange of the electron beam. Conversely, as the shiel-d is moved backtoward reference point 24 shield aperture 34a may be reduced in size toaccommodate the same scanning excursion of the beam. In practicing theinvention, the position of the shield and the diameter of aperture 34aare such that the electron .beam of the tube, in response to thedeflection eld of yoke 28, scans on screen 13 a pattern having a maximumdimension equal to the diagonal measure of the rectangular faceplate. Anelectron beam which is overscanned loutside of this limit is of courseintercepted by the shield as indicated by lines 52. Shield 34 isdiscontinuous, including gap 38 shown in FIGURES 2 and 3 to prevent theestablishment of eddy currents. However, an offset portion 40 overlapsthe gap and simulates a continuous intercepting surface for the shield.

A getter ring 37 is attached to the side of shield 34 which faces thescreen area.

Dual centering means are pnovided for centering the electron beam inaperture 34a of shield 34 and in yadldition, for centering the scanningraster on screen 13. The first centering means includes a pair ofpermanent ring magnets 41, 41 which encircle neck 11 and are received insupport frame 33t. This is a well-known arrangement for centering theelectron beam in Iaperture 34a. An additional pair of ring magnets 42,42 are `afiixed to yoke 28 and encircle funnel section 12 of the tube tocenter the raster `on the screen. The ring magnets themselves are wellknown in the art, each ring being polarized with a north and south poleat diametrically opposed positions on the ring. When the rings of anypair are rotated relative to one another to have their respective northand south poles adjacent, essentially zero deflection is imparted to theelectron beam; conversely, when `all poles are adjacent a maximumdeflecting force is produced.

In operating the described tube structure, after the tube has beenenergized its electron beam is centered in aperture 34a by manipulationof rings 4l1, 41. Thereafter, the scanning r-aster, which results fromthe influence of the deflection fields of yoke 28 upon the beam, iscentered ron screen 13 by manipulation of the remaining pair ofcentering magnets 42, 4t2. Of course, these latter centering magnets maybe replaced by a controlled D.C. centering current supplied to thedeflection yoke. Having thus adjusted the scanning, the tube is operatedin conventional manner but it will be found that there is a significantimprovement in contrast. The aperture 34a of shield 34 causes thescanning pattern on screen 13 to` cover the rectangular image area ofthe faceplate and to have a maximum dimension equal to the diagonaldimension of the faceplate. As a consequence, overscanning, especiallyat the corners of the raster, is avoided since electrons of the beamthat would accomplish overscanning such as those represented by lines 52are, in fact, intercepted by shield 34. By thus confining the sc-ansionof the beam, particularly at the corners of the faceplate, the strayelectrons, both primary and secondary electrons, which heretofore havediluted contrast, are greatly minimized because the beam does not nowimpinge against portions of the envelope which give rise to such strayelectrons.

In addition to improving contrast, the described structure is free ofthe difficulties attendant the use of shields having rectangular-shapedapertures. The described structure, featuring a shield with a circularaperture, greatly facilitates the production problems and minimizes thecost of the tube. Moreover, since the shield may be mechanicallysupported from the gun assembly and may have overall dimensions thatpermit its being positioned within the neck yof the tube, the inventionis applicable to monochrome tubes and does not require that thefaceplate be severable from the funnel section of the envelope.

While a particular embodiment of the invention has been shown anddescribed, it will be obvious to tho-se skilled in the yart that changesand modifications m-ay be made without departing from the invention inits broader aspects, and, therefore, the aim in the appended claims isto cover all such changes .and modifications las fall within the truespirit and scope of the invention.

We claim:

1. A cathode ray tube having a beam deflection eld with a predeterminedpoint at which beam deflection is initiated, `said tube comprising:

an evacuated envelope including la neck, a funnel,

and a substantially rectangular-shaped faceplate having an image screenthereon;

an electron gun within said neck f-or projecting an electron beam alonga predetermined path, through said deflection field, toward said screen;

and an electron shield having a maximum ldiameter less than the diameterof said neck and having a circular aperture, supported by said gun withsaid shield on the screen side of said predetermined point and with theaperture thereof disposed across said path and substantially cO-axialwith said envelope funnel, the position of said shield and the diameterof said Iaperture being such that the deflected electron beam iseffectively shadowed from the initially flared portion of said funneland is confined to scanning on said screen a pattern having a maximumdimension equal to the diagonal measure of said rectangular faceplate.

2. A cathode ray tube having a beam deflection field with apredetermined point at which beam deflection is initiated, said tubecomprising:

an evacuated envelope including a neck, a f-unnel, and

a substantially rectangular-shaped faceplate having an image screenthereon;

an electron gun within said neck for projecting an electron beam `alonga predetermined path, through said deflection field, toward said screen;

`an electron shield having a maximum diameter less than the diameter ofsaid neck and having a circular aperture, supported by said gun withsaid shield on the screen side of said predetermined point and with theaperture thereof disposed across said path and substantially co-axialwith said envelope funnel, the position of `said shield and the diameterof said aperture being such that the deflected electron beam iseffectively shadowed from the initially flared portion of said funneland is confined to scanning on said screen a pattern Ihaving a maximum`dimension equal to the `diagonal measure lof said rectangularfaceplate;

and a pair of independently adjustable centering means for centeringsaid beam relative to the aperture of said shield and for centering saidscanning pattern relative to said screen.

3,334,253 6 3i. A cathode nay tube having a beam deflection field tostimulate a continuous barrier for electrons exwith a predeterminedpoint at which beam deflection is inicept for the aperture thereof.tiated, said tube comprising: 6. A cathode ray tu-be having Ia beamdeflection field an evacuated envelope including a neck, a funnel, andwith a predetermined point at which beam deflection is a substantiallyrectangular-shaped faceplate having an image screen thereon; y

an electron gun within said neck for projecting an electron beam along apredetermined path, through said deection field, toward said screen;

initiated, said tube comprising:

an evacuated envelope including a neck, a funnel, and a substantiallyrectangular-shaped faceplate having an image screen thereon;

an electron Igun within `said neck for projecting an electron beam alonga predetermined path, through said deflection eld, toward said screen;

and a conductive electron shield having a maximum diameter less than thediameter of said neck Iand having a circular aperture and a slotextending from said aperture to the outer periphery of said shield,supported by said gun with said shield on the screen side of saidpredetermined point and with the aperture thereof disposed across saidpath and substantially co-axial with said envelope funnel, the positionof said shield and the diameter of said aperture being such that thedeflected electron beam is effectively shadowed from the initiallyflared portion of said funnel and is confined to scanning on said screena pattern having a maximum dimension equal to the a conductive electronshield having a maximum diameter less than the diameter of said neck andhaving a circular aperture and a slot extending from said aperture tothe outer periphery of said shield, supported by said gun with saidshield on the screen side 5 initiated, said tube comprising:

'an evacuated'envelope including a neck, a funnel, and a substantiallyrectangular-shaped faceplate having an image screen thereon;

an electron gun within said neck for projecting an elecan electronshield having a maximum diameter less than lo tron beam along apredetermined path, through said the diameter of said neck and having acircular apdeection field, toward said screen; erture, SuPPorted 'hySaid gun With Said Shield 0n a conductive electron shield having -amaximum diamthe Sereen Side of Said predetermined Point and With eterless than the diameter of said neck and having the aperture thereofdiSPoSed aeroSS Said Path and a circular aperture and a slot extendingfrom said s-ubstantially co-axial with said envelope funnel, theaperture to the outer periphery of said shield, supposition of SaidShield and the diameter of Said ported by said gun withsaid shield onthe screen side aperture being such that the deflected electron beam 0fSaid predetermined point and with the aperture iS etfeetiVelY shadowedfrom the initially ilared Por' thereof disposed across said path andsubstantially tion of Said funnel and iS ooniined to Seanning onco-axial with said envelope funnel, the position of Said Sereen aPattern haVing a rnaXirnunl dimension said shield and the diameter ofsaid aperture being equal t0 the diagonal measure -of Said rectangularsuch that the deiiected electron bea-m is effectively faceplate;shadowed from the initially dared portion of said and a first centeringdevice positioned on the gun side of funne1 and is confined t0 Scanningon said screen a Said predetermined Point for eentering Said hearnpattern having a maximum dimension equal to the relative to the aperture0f Said Shield and a Second 25 diagonal measure of said rectangularfaceplate; centering device positioned on the screen side of said amember overlapping Said slot to cause said shield Shield for centeringSaid Soanning Pattern relatiVe to simulate a continuous barrier forelectrons except t0 Said SCreenfor the aperture thereof;

4- A Cathode 'ray tube having 'a beam deflection 'field :and a getterring having an internal diameter substanwith a predetermined point atwhich beam dellection is tially larger than the diameter of saidaperture, supported by said gun in coaxial alignment with and on thescreen side of said shield. 7. A cathode ray tube having a beamdeilection field with a predetermined point at which beam deilection isinitiated, said tube comprising:

an evacuated envelope including a neck, a funnel, and a substantiallyrectangular-shaped faceplate having :an image screen thereon;

an electron gun within said neck for projecting an electron beam along apredetermined path, through said deflection field, toward said screen;

an electron shield having a circular aperture having a maximum diameterless than the diameter of said neck and, supported in said neck of saidenvelope by said gun with said shield on the screen side of saidpredetermined point and with the aperture thereof disposed across saidpath and substantially co-axial with said envelope funnel, the positionof said shield and the diameter of said aperture being such that thedeflected electron beam is eiectively shadowed from diagonal measure ofsaidrectangular faceplate. the initially flared portion of said funneland is con- .5 A cathode lay tube, llavmg `a beam dellectloll lleldfined to scanning on said screen a pattern having a with a predeterminedpoint at which beam deflection is maximum dimension equal to thediagonal measure initiated, said tube comprising: of Said rectangularfaceplate;

all evacuated. envelope lllcllldlllg a neck a funnel dnd 55 and a pairof independently adjustable centering means a s ubstalltlallylectallgular'shaped faceplate havmg for centering said beam relative tothe aperture of :anlmage Screen.th.eleol.li said shield and forcentering said scanning pattern an electron gun Within said neck forprojecting an elecrelative to said Screen.

troll bam along a Pledelelmllled Path through Sad 8. A cathode ray tubehaving a beam deflection field dellectlon eld lowald Sall Screen; with apredetermined point at which beam deflection is initiated, said tubecomprising:

an evacuated envelope including a neck, a funnel, and a substantiallyrectangular-shaped faceplate having an image screen thereon;

an electron gun within said neck for projecting an 0f Sad predeterminedPoldi and With the aperture electron beam along a predetermined path,through thereof disposed across said path and substantially Saiddeflection eld toward said screen; eo-aXial With Said envelope funnel,the Position of an electron shield having a circular aperture having aSaid Shield and the diameter 0f Said aperture being maximum diameterless than the diameter of said such that the deflected electron beam iseffectively neck and, supported in said neck 0f said envelope shadowedfrom the initially dared portion of said by said gun with said shield onthe screen side of funnel and is confined to scanning on said screen asaid predetermined point and with the aperture therepattern having amaximum dimension equal to the of disposed across said path landsubstantially co-axial diagonal measure of said rectangular faceplate;with said envelope funnel, the position of said shield and a memberoverlapping said slot to cause said shield and the diameter of saidaperture being such that the deected electron beam is effectivelyshadowed from the initially flared portion of said funnel and isconfined to scanning on said screen a pattern having a maximum dimensionequal to the vdiagonal measure of said rectangular faceplate;

and a rst pair of centering magnet rings positioned on said neck on thegun side of said predetermined point for centering said beam relative tothe aperture of said shield and a second pair of centering `magnet ringspositioned on said funnel of said neck on the screen side of said shieldfor centering said scanning pattern relative to said screen.

References Cited UNITED STATES PATENTS JAMES W. LAWRENCE, PrimaryExaminer.

10 v. LAFRANCHI, Assistant Examiner.

1. A CATHODE RAY TUBE HAVING A BEAM DEFLECTION FIELD WITH APREDETERMINED POINT AT WHICH BEAM DEFLECTION IS INITIATED, SAID TUBECOMPRISING: AN EVACUATED ENVELOPE INCLUDING A NECK, A FUNNEL, AND ASUBSTANTIALLY RECTANGULAR-SHAPED FACEPLATE HAVING AN IMAGE SCREENTHEREON; AN ELECTRON GUN WITHIN SAID NECK FOR PROJECTING AN ELECTRONBEAM ALONG A PREDETERMINED PATH, THROUGH SAID DEFLECTION FIELD, TOWARDSAID SCREEN;